In ADSL (asymmetric digital subscriber line) systems based on DMT (discrete multi-tone) modulation, an incoming serial bit-stream is typically divided into parallel streams, which are used for performing quadrature amplitude modulation (QAM) on the various tones. After modulation is performed with an inverse fast Fourier transform (IFFT), a cyclic prefix (CP) is added to each symbol. If the prefix is longer than the channel impulse response (CIR), demodulation can be implemented by passing the received signal through an FFT followed by a complex 1-tap frequency-domain equalizer (FEQ) for each tone in order to compensate for channel amplitude and phase effects.
A long prefix, however, results in large overhead with respect to the data rate. One approach to this problem is to insert a (real) time-domain equalizer (TEQ) before the demodulation phase in order to shorten the channel impulse response to fit within the CP. One perceived shortcoming with this approach, however, is that if the channel is “imperfectly” shortened where the CIR does not fit within the CP, inter-symbol interference (ISI) between two successive symbols and inter-carrier interference (ICI) between different carriers arises. As such, those skilled in the art will appreciate that there is a need for addressing the undesirable side effects of an imperfectly shortened channel.